Do NOT check if corresponding mappings are already defined to define default mappings

[lambdalisue]

If we check whether the LHS exists in the mapping, users who remap <C-n> to something like <Up> would not have Fall’s default mappings defined, making it very inconvenient to use. Additionally, rather than checking if the RHS is already registered, we can allow users to define their mappings after the default mappings by setting the timing accordingly. This way, users can remove default mappings using <Nop>. Therefore, I modified the specification to add internal events such as ~System to enforce the timing.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.72%. Comparing base (51f9bf0) to head (e7e4455).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
denops/fall/util/emitter.ts	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   80.67%   80.72%   +0.04%     
==========================================
  Files          24       24              
  Lines        1651     1655       +4     
  Branches      163      163              
==========================================
+ Hits         1332     1336       +4     
  Misses        316      316              
  Partials        3        3              

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